fix(bench): make can_reset_vm_hwm side-effect-free with open probe (closes #34)

crates/bench/src/metrics.rs

@@ -80,9 +80,12 @@ impl MetricsCollector {
         })
     }
 
-    /// Check if we can reset VmHWM (i.e., have root)
+    /// Check if we can reset VmHWM (i.e., can open `/proc/self/clear_refs` for writing)
     pub fn can_reset_vm_hwm() -> bool {
-        fs::write("/proc/self/clear_refs", "5").is_ok()
+        std::fs::OpenOptions::new()
+            .write(true)
+            .open("/proc/self/clear_refs")
+            .is_ok()
     }
 
     /// Get file inode number